In your Appearance Screen, you will see a list of currently installed themes, including the WordPress Twenty Twenty-One theme. To quickly change the theme, simply click on the Activate button under one of the themes listed, then click on your site name in the top toolbar to see how it looks. Go back to Appearance > Theme and click the Activate button under the WordPress Twenty Twenty-One theme to bring the design back to what you had.
You can read more here: https://wordpress.org/support/article/first-steps-with-wordpress/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath